The present invention relates to a control apparatus for a Diesel engine, which has a fuel metering device of the pneumatic type for metering fuel to be injected by a fuel injection pump to the engine in response to the flow rate of intake air and an auxiliary intake throttling device actuable only during light load operation of the engine (especially in case of the engine idling) in order to decrease the level of engine noise.
It is well known that the level of Diesel engine noise is considerably higher than the level of noise produced by spark-ignition engines. Especially in case of idling, noise produced by Diesel engines gives annoyance not only to the drivers but also people around the Diesel-engine-mounted vehicles. To overcome this problem, it has been proposed to throttle intake air. In practice at high engine speeds the level of noise produced when intake air is throttled is almost equal to the level of noise produced when intake air is not throttled at all, and only at low speeds there is a difference in level of noise between the both cases. In other words, the method of throttling intake air is effective in reducing the level of noise only at low engine speeds, but is virtually ineffective at high engine speeds. In addition the method of throttling intake air in order to reduce the engine noise gives rise to some problems. That is, when intake air is throttled at high engine speeds or under increased load condition, the engine output drops and the color of emission gases is worsened.
Therefore the device for throttling intake air which is effective in practice must be actuable only during light load operation in which the device is very effective in reducing the engine noise without causing any adverse effect on the engine output.
However when the intake air throttling device of the type described is incorporated with a fuel injection pump or pneumatic governor, the latter does not function properly when the intake air throttling device is actuated because the negative pressure at the throat of an auxiliary venturi which is transmitted to the negative pressure chamber in the pneumatic governor is varied when the intake air is throttled.